Optical fibres have traditionally been installed into underground ducts by attaching a pulling member to one end of the cable, and winching the cable into the duct. As a result, such cables were large and heavily reinforced to protect the relatively delicate optical fibre elements from damage during installation.
Traditional cables were constructed by first manufacturing sub-assemblies comprising tubes manufactured from thermoplastic materials and containing typically twelve fibre optic elements. A number of these tubes were then assembled together by stranding them around a central strength member. The stranding process, and the fact that the tube is large relative to the space occupied by the fibre optic elements, means that all fibres experience the same strain when the cable is bent during installation, and the loose tube construction allows the fibres to move and accommodate the strain, resulting in minimal signal losses.
More recent techniques for cable installation involve blowing the cable into a duct by means of compressed air, for example as described in EP 0108590. This blowing process distributes the installation force along the entire length of the cable within the duct, as a result of which the installation force at the leading end of the cable can be reduced, and much of the reinforcement can therefore be removed from the cable. This provides significant advantages, since there is an increasing requirement for cables to become more compact, primarily because city networks are congested and providing new underground ducts in cities is expensive and involves substantial disruption.
Installation of cables by blowing involves both the use of fluid drag operating on the sheath of the cable, and a pushing force, usually generated by drive rollers or a caterpillar pushing device which forms part of the blowing equipment. At the initial stages of installation, there is very little cable installed in the tube, and the effect of fluid drag is therefore small compared to the pushing effect. As more of the cable is installed, the installation force derived from the fluid drag becomes more significant.
It is therefore desirable for cables designed for installation by blowing to have adequate stiffness to facilitate the initial pushing requirement. In the case of cables constructed from sub-assemblies, the fibres are loosely contained in an outer sheath of the sub-assembly. Because the individual fibres are not constrained, they do not provide the cable with sufficient stiffness, and it is therefore desirable that the cable be constructed with a central strength member, typically manufactured using a glass-reinforced polymer. The strength member is sufficiently stiff that it dominates the stiffness of the assembly and, because of its central location, ensures that the cable does not preferentially bend in one direction rather than another.
However, the use of a central strength member undesirably increases the size of the cable.
An attempt to produce a cable for installation by blowing without the use of a central strength member is disclosed in EP 0521710, which describes a cable in which 2, 4 or 8 individual optical fibres are in touching contact and are encapsulated in an outer layer, typically a UV cured acrylate. Encapsulation of the fibres in a UV cured acrylate results in the individual fibres being restrained from moving relative to each other, and the cable derives its stiffness from this, eliminating the requirement for the central strength member. However, the fact that the fibres are locked together means that when the assembly is bent, the fibres impose a strain on the outer coating of the cable. The larger the diameter of the fibre unit, the greater the tensile stress applied to the outer surface for a given bend radius. Fibre optic cables containing 4 or 8 fibres are found to create such a high load that a phenomenon known as fibre breakout is experienced, and which has a detrimental effect on cable performance.
EP 0521710 discloses a process which produces satisfactory results on cables with fibre counts of 2, 4 and 8 fibres by changing the coating arrangement to ensure that fibres do not break out of the coating, even with larger diameter cables containing 8 fibres. However, it is desirable to manufacture cables having more than 8 fibres, but attempts to manufacture such cables have had difficulty in overcoming the problem of fibre breakout. An attempt to overcome this problem is disclosed in EP 0422764 in which 12 fibres are provided, the fibres being accurately located and locked in position relative to each other by first assembling sets of 4 fibres into a ribbon sub-assembly by edge bonding the 4 fibres to each other, and laying 3 such sub-assemblies on top of each other to form a basic construction which is then encapsulated in an outer layer.
Compact ribbon cable assemblies of this type suffer from the drawback that the surfaces of the ribbons in such cables are smooth, and the ribbons are therefore free to slide relative to each other. In addition, because the fibres are bonded in a flat arrangement, when the cable is bent in a direction which imposes a sideways moment on the flat ribbons, the force generated is high and the central ribbon, which is free to slide between the two outer ribbons, is then forced to break out through the outer acrylate coating, producing micro bending and unacceptable signal losses.
An attempt to overcome this problem is disclosed in DE 4211489 by reducing the diameter of the individual optical fibres. An individual fibre is provided with a protective outer layer of 25 microns or less, instead of the 60 micron coating usually applied. This reduces the overall diameter of the individual fibres by approximately 30%, which has the effect of making the assembly smaller and therefore reducing the strain imposed on the coating. However, this arrangement is inconvenient because most commercially available fibres have the same dimensions, and equipment for splicing and terminating fibres is therefore adapted to these standard dimensions. Furthermore, DE 4211489 describes an arrangement in which adjacent fibre ribbons are offset to reduce the height of the assembly. Such ribbon constructions produce assemblies with a very high preference to bend in one direction, and are therefore not suitable for cables designed for installation by blowing.
U.S. Pat. No. 5,787,212 discloses an arrangement of 7 fibres of equal diameter in which 6 fibres are disposed in a circular pattern in touching contact with each other and around a central fibre. When the fibres are coated with resin curable by UV radiation, the touching fibres ensure that resin does not enter the spaces between the fibres, which minimises the problem of UV light not adequately penetrating the outer fibres and inadequately curing resin located between the fibres. Uncured resin has the potential to break down and generate agents which may damage the glass fibres, adversely affecting their long-term signal transmitting performance.
Although the arrangement of U.S. Pat. No. 5,787,212 has very good bending properties, since it is completely balanced with no preferential bending characteristics, and strain imposed on one fibre is partially distributed into the other fibres by virtue of the touching contact, groups of 7 fibres are not used commercially, since fibres are almost always deployed in pairs and it is desirable to manufacture cables with higher fibre counts suitable for installation by blowing. Traditional cables almost exclusively contain 12 fibres or multiples thereof.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.